Air pollution, specifically the tiny particles known as PM2.5, is linked to higher rates of dementia, according to a recently published study in The Lancet.

Nitrogen dioxide (NO2) and black carbon, which is the black soot left behind when combustion is incomplete, have also been linked to higher risk of dementia in the study, which is headed by researchers at UK’s Cambridge University.

“What this means is that cleaner air policies, including those targeting diesel, could help protect brain health, not just lung and heart health. Dementia is a devastating disease, and while we wait for a cure, we need to act on modifiable risk factors. Air pollution is one of the biggest,” Haneen Khreis, one of the authors of the study, told Health Policy Watch.

Researchers reviewed 51 existing studies across several countries up to October 2023 to arrive at this conclusion.

PM2.5 is 1/28 of the width of a human air and much of it is released during the burning of gasoline, oil, diesel or wood. PM10 particles are relatively larger, though still invisible to the naked eye.

“PM2.5, or fine particulate matter, can cross the blood-brain barrier, resulting in inflammation and disruption of brain function. There is also evidence to suggest that fine particles can travel through the olfactory nerve into the brain,” Pallavi Pant, who is the head of global initiatives at Health Effects Institute told HPW.

“Exposure to air pollution may also have impacts on brain development and functioning in children, including an increased risk for neurodevelopmental disorders like autism and psychological disorders like anxiety and depression,” Pant pointed out.

The dangers of PM2.5

Air pollutants are categorized by their sizes. There is PM2.5 and there is PM10. Evidence does not yet link PM10 with higher rates of dementia, but the number of studies looking closely at PM10 were small, the researchers concluded.

Gases like NO2, and particles the size of PM2.5 or less, are dangerous because they are small enough to enter the bloodstream from the lungs after being inhaled.

Once inside the body, they can travel from head to toe, according to Palak Balyan, research lead at Climate Trends, headquartered in New Delhi.

There are two ways patients can develop dementia, a broad term describing a decline in mental abilities severe enough to interfere with a person’s everyday life, says Balyan.

One is a natural consequence of ageing, while the other is caused by blockage in the brain – including by air pollution.

“These small particles (PM2.5) block a lot of arteries, veins or small capillaries in our brain. That also leads to dementia,” she said.

Black carbon

While PM2.5 and PM10 do get some attention, black carbon does not.

“Most studies have focused on PM2.5 and NO₂, but we need much more attention on black carbon, given its major role in both health harms and climate change. These pollutants mainly come from car exhaust, power plants, industry, and diesel engines,” said Khreis, who added that black carbon or soot is sometimes smaller than PM2.5 and sticky.

“If that small sized black carbon particle sticks inside your lung or inside any other capillary in your body, that can create more damage than any other bigger sized particle,” Balyan explained.

Limited evidence from Global South

Of the 51 studies that were examined, 20 (39%) were done in Europe and 17 (33%) in North America, representing more than half of the overall evidence.

“Most of the data comes from high-income countries, and often from White, urban populations. That means we’re missing critical evidence from low- and middle-income countries, and from groups that face the highest exposures because of structural inequalities,” Khreis said.

She added that the global burden is probably underestimated and the risk in some groups within urban areas is concealed.

A majority of the world’s most polluted cities are in the developing countries. In 2024, 49 of the world’s most polluted cities were in Asia and one in Africa, according to data from IQAir, a Swiss air monitoring company.

But only 12 studies (24%) from Asia were included and none from Africa or Latin America. Two (4%) others were from Oceania (both in Australia).

Some of the earliest studies linking air pollution and impacts on the brain were conducted in Mexico City. Dogs living in polluted environments in Mexico City had more neurodegeneration than dogs living in cleaner environments outside Mexico City, according to one study. Similar studies were also done with children going back to 2008.

Evidence from India also suggests that those using polluting sources of cooking fuel were at a higher risk of cognitive impairment, especially rural women, given that they tend to have higher exposure to polluting cooking fuels like firewood.
“A majority of studies are currently from Europe or North America, or China, and we need a broader global evidence base representing other regions,” Pant said.

“Having said that, with the available evidence, the case for addressing air pollution to help reduce the dementia burden at the population level is strong,” she added.

A ‘modifiable risk factor’ for dementia

The Lancet Commission has included air pollution as a modifiable risk factor for dementia in 2024, as it is possible to improve air quality.

Balyan sees some positive trends in addressing air pollution, including more awareness, more funding for research on air pollution and more collaboration.

“Now engineers and doctors are collaborating, they are working together. So that kind of collaboration has also increased which is leading to more number of studies. International collaboration has also increased because of this easy to work online system,” she said.

Dr Maria Neira, the outgoing director of the World Health Organization’s (WHO) Department of Environment, Climate Change and Health since 2005, is widely credited with increasing awareness about the impact of air pollution on health.

Image Credits: WHO, U.S. Environmental Protection Agency (EPA), WHO, US Mission Geneva .